1. Field of the Invention
The present invention relates to reducing packet loss during multicast data transmission in a wireless communication environment, and more particularly, to a reliable multicast data retransmission method by grouping wireless terminals in a wireless communication environment and performing multicast data retransmission for a particular group of wireless terminals which needs multicast data retransmission, thereby providing reliable multicast data retransmission and improving the efficiency of a network.
2. Description of the Related Art
To allow for data transmission over Internet, unicast, broadcast, multicast, and anycast transmissions have been provided.
FIG. 1 is a view for describing unicast, broadcast, and multicast transmissions.
Unicast transmission is a one-to-one type of data transmission that permits one sender to transmit data to one receiver. Consequently, when the sender desires to transmit data to multiple receivers, the unicast transmission overloads sender resources and exhibits inefficient network bandwidth usage. Moreover, the unicast transmission is not available when data to be transmitted to the multiple receivers is bulky such as multimedia data.
Broadcast transmission is a one-to-many type of data transmission that permits a sender to transmit data to all of the receivers connected to a network. Once the sender transmits data to one receiver, the rest of the receivers receive duplicates of the data. Consequently, the broadcast transmission can improve the efficiency of the network and reduce network congestion. However, unnecessary traffic is created because data is transmitted even to a receiver that does not desire to receive the data.
Anycast transmission has recently been introduced, in which data is transmitted to the most appropriate server having an anycast address. The anycast address is used to define each group of servers that provide the same service to a user. The user with need of data selects the most appropriate server capable of providing the best service from among a group of servers having the same anycast address and is provided with the data.
In multicast transmission, the sender transmits data only to a multicast group including receivers that desire to receive the data. Since the multicast transmission supports a one-to-many type of data transmission and a many-to-many type of data transmission, it eliminates the disadvantages of the unicast transmission and broadcast transmission. The sender can improve the efficiency of the network and reduce network congestion because the sender is required to transmit only a single data packet. Also, the receiver can prevent the creation of unnecessary traffic because the receiver is required to provide duplicates of the-data packet only to the multicast group.
Multicast transmission is the most efficient method for effectively transmitting data over an Internet broadcast, a video on demand (VOD), or an Internet videoconference.
In particular, multicast transmission exhibits a more efficient performance in a wireless communication environment having limited network resources such as bandwidth and power. However, when a multicasting technique for use with a fixed network such as a cable network is used in a wireless communication environment, a separate multicast tree must be constructed for each network source, which increases control traffic and causes frequent packet loss.
Accordingly, a method that maintains the advantage of multicast transmission and is adequate for a wireless communication environment is needed for an efficient communication system.
Retransmission is a key mechanism of reliable multicast data transmission to recover packet loss. By using acknowledgement (ACK) frame transmission, the sender can determine whether the receiver accurately receives data and retransmit, if necessary, the data based on the determined result.
However, when the sender receives acknowledgement (ACK) messages from multiple receivers, a delay in receiving the ACK messages from all of the receivers is longer than the amount of time necessary for data transmission, which disrupts efficient usage of network resources. In an attempt to solve such a problem, negative acknowledgement (NACK) frame transmission instead of ACK frame transmission is used for retransmission. However, such an attempt has not been useful for the wireless communication environment having a high packet error probability.
Instead of automatic repeat request (ARQ) using ACK or NACK frame transmission, data may be transmitted after being replicated. Since the sender does not need to wait for additional information from the receiver, it is possible to maintain a constant efficiency of a channel irrespective of the number of receivers and reduce the packet loss probability due to repeated transmission.
However, since a status of the channel is dynamically changed over time in the wireless communication environment, a repeated transmission in a fixed way cannot help the recovery of packet loss. In other words, since the position and status of a channel of each receiver are different from one another, the receiver with a superior channel can receive all data packets from the sender all the time, and on the other hand, the receiver with an inferior channel has the high packet loss probability all the time.
Therefore, the following aspects have to be considered to achieve efficient data transmission in the wireless communication environment. First, retransmission has to be performed in consideration of the status of the channel of each receiver. Second, retransmission must not be performed on a terminal with a channel inferior to that of the terminal with the worst data reception status from an access point (AP). This is because such retransmission can improve the reliability of data transmission, but may result in inefficient usage of network resources. Third, retransmission has to allow for reliable multicast data transmission and efficient usage of network resources.